If a current can make a magnetic field (an electromagnet), can a magnet make a current? In 1831 Michael Faraday found that it can, but only while something is changing. That discovery runs almost every power plant in the world.
Key idea
Moving a magnet near a coil, or a coil near a magnet, induces a current in the coil. The current flows only while the magnetic field through the coil is changing. Faster motion, a stronger magnet or more turns make a bigger current. A generator uses this to turn motion into electrical energy.
Why it works
Push a magnet into a coil and the field inside the coil grows, so a current flows. Hold it still and nothing changes, so the current stops. Pull it out and the current flows the other way. In a generator, a coil spins between magnets, so the field through it changes all the time, and by conservation of energy, the electrical energy comes from the work done turning the coil.
Worked example: Magnet and coil
A magnet is pushed into a coil, held still inside it, then pulled out. When does the meter show a current?
- Pushing in: the field in the coil grows, so a current flows.
- Held still: no change, no current.
- Pulling out: the field shrinks, so a current flows the other way.
Answer: Only while the magnet moves; opposite ways in and out
Worked example: Where the energy comes from
A wind turbine's blades turn a generator. What energy change happens?
- The wind's kinetic energy turns the blades.
- The generator turns that motion into electrical energy (plus some heat).
Answer: Kinetic energy into electrical energy
Common mistake
Thinking a magnet resting inside a coil makes a steady current. A current is induced only while the field through the coil is changing. A still magnet gives a steady field and no current. Check: watch the meter while the magnet is still.
In most power plants, steam, falling water or wind spins a turbine connected to a generator. The current in your outlets swings back and forth 60 times each second (60 Hz) in North America.